Spanner is an adjective. One would listen back to an especially audacious bit of vocal expression and say, almost with an air of awe and admiration, that that was really spanner.

And it was. (John)

I'm not too certain about the "awe and admiration" bit. More like tooth grinding embarassment. (Ben)

I think Mr Eliot puts it well (Gerard)...

Between the idea
And the reality
Between the motion
And the act
Falls the Spanner

Between the conception
And the creation
Between the emotion
And the response
Falls the Spanner

Between the desire
And the spasm
Between the potency
And the existence
Between the essence
And the descent
Falls the Spanner

See also: Is Mother Invisible? and Rota